multifilesystem: Improve performance of PROPFIND

The PROPFIND does not need to check the filesystem for the file existence,
because the list comes from the directory listing, similar as the other
parts already do it. That was highly visible with large collections (10k+ items),
causing massive delays.

Also avoid repeated stat() calls on the same path.

CHANGELOG.md

@@ -1,6 +1,8 @@
 # Changelog
 
 ## 3.8.0.dev
+* Fix: storage/multifilesystem: depth:1 PROPFIND no longer re-runs the filesystem collision check (path_to_filesystem) for every item in a collection; this made listing large collections O(n^2) on file systems not detected as collision-free
+* Improve: storage/multifilesystem: avoid redundant stat() calls per item in get/upload when use_mtime_and_size_for_item_cache is enabled
 
 ## 3.7.8
 * Fix: time-range filter on a VTODO having DTSTART/DUE and also CREATED/COMPLETED used the CREATED->COMPLETED duration instead of the DTSTART->DUE one, so completed tasks were missing from (or wrongly returned by) calendar-query REPORT results